Abstract

Cortical information flow may be altered by maturational changes in use-dependent synaptic dynamics at intracortical connections. Using multi-neuronal whole-cell voltage recordings, we characterized the development of synaptic dynamics at excitatory connections between layer 5 pyramidal neurons in visual cortex during the first 4 postnatal weeks (n = 158 pairs). In young (P11-15) cortex, unitary EPSPs were large and reliable, with a median amplitude of ~ 570 uV and a median failure rate of only 9%. In animals P25-29, the median uEPSP amplitude had decreased markedly to ~ 135 uV, accompanied by an increased coefficient of variation and uEPSP failure rate (34%). Some of these developmental changes in synaptic properties (i.e. increased uEPSP failures) were temporally associated with postnatal eye opening. Mature layer 5 connections showed strong, frequency-dependent paired pulse facilitation across the range of stimulation frequencies tested (10-50 Hz, mean paired pulse ratios between 1.3 and 1.95). In younger cortex, paired pulse depression was observed across the frequency range (mean paired pulse ratios between 0.53 and 0.8) and less variability in paired pulse dynamics was observed. Developmental modification of synaptic dynamics was also manifest during complex action potential trains; P25-29 synapses effectively maintained transmission during prolonged spike trains across a range of frequencies, whereas synaptic potentials in P11-15 cortex depressed rapidly within a few action potentials, except at very low stimulation frequencies (0.2 Hz). Thus, the first postnatal month sees a reduction in the efficacy of transmission of single action potential signals between Layer 5 pyramidal cells in visual cortex, accompanied by improved dynamic range and capacity for transmission of complex spike trains.